Field of the Invention
The invention concerns a method to determine a magnetic resonance control sequence with a pulse arrangement that acts selectively in at least two spatial directions in order to excite a limited, rotationally symmetrical excitation profile within an examination subject. Moreover, the invention concerns a method to operate a magnetic resonance system with such a magnetic resonance control sequence; a method to calibrate a magnetic resonance system using such a method; a control sequence determination device in order to determine such a magnetic resonance control sequence; and a magnetic resonance system with such a control sequence determination device.
Description of the Prior Art
In a magnetic resonance tomography system (also called a magnetic resonance system), the body to be examined is typically exposed to a relatively high, optimally homogeneous basic magnetic field (B0 field)—for example of 1, 5, 3 or 7 Tesla—with the use of a basic field magnet system. A magnetic field gradient is additionally applied by a gradient system. Radio-frequency excitation signals (RF signals, or also called RF excitation pulses or just RF pulses) are then emitted by suitable antenna devices via a radio-frequency transmission system, which causes the nuclear spins of specific atoms to be excited to resonance by a defined “flip angle” i.e., the spins are deflected relative to the magnetic field lines of the basic magnetic field. Upon relaxation of the nuclear spins, RF signals (magnetic resonance signals) are radiated that are received by suitable reception antennas and then are processed further. Finally, the desired image data are reconstructed from the raw data acquired in such a manner.
For a defined measurement, a pulse sequence is to be emitted that includes a radio-frequency pulse sequence to be emitted and a gradient pulse sequence to be (switched) activated in coordination with the RF pulse sequence (with time-coordinated gradient pulses in the slice selection direction, in the phase coding direction, and in the readout direction). In particular, the timing within the sequence—i.e. in what time intervals which pulses follow one another—is significant for the imaging. A number of control parameter values are normally defined in what is known as a measurement protocol, which is created in advance and retrieved (from a memory, for example) for a defined measurement, and can be modified as necessary by the operator on site, who can provide additional control parameter values, for example a defined slice spacing of a stack of slices to be measured, a slice thickness, etc. A magnetic resonance control sequence or pulse sequence is then calculated on the basis of all of these control parameter values.
In conventional procedures, the acquisition of images of the inside of the subject takes place slice-by-slice. A relatively thin, planar slice—typically between 1 and 10 mm—is selectively excited. Such a selective excitation is achieved by activating a gradient in the slice selection direction in coordination with the radio-frequency excitation pulse. Such a pulse arrangement, composed of the exciting radio-frequency pulse and the associated gradient, causes the resonance condition to be satisfied only in a slice orthogonal to the slice selection direction. The thickness of the excited slice in the slice selection direction is determined by the amplitude of the slice selection gradient and the frequency bandwidth of the radio-frequency pulse. The excited slice can be displaced (shifted) along the slice selection direction by a “shift” (a displacement) of the carrier frequency of the radio-frequency field. The selection volume of these one-dimensional, selective RF pulses is limited only in the direction orthogonal to the slice plane. This slice selection direction often proceeds parallel to what is known as the z-axis (the longitudinal axis of the tomography scanner), and thus also parallel to the longitudinal axis of a patient lying in the scanner. A spatial coding within the slice then takes place by a phase coding in one direction (most often the y-direction) and a readout coding in the second direction (most often the x-direction). In this way, a two-dimensional spatial frequency domain (known as k-space) in which the raw data are entered, is filled. An image of the slice is created from the data entered into k-space by a two-dimensional Fourier transformation thereof.
Moreover, multidimensional selective RF pulses are known. For example, a two-dimensional selective RF pulse can select (selectively excited nuclear spins in) a long rod or cylinder that is spatially limited in both directions orthogonal to the rod axis, thus in the radial direction orthogonal to the cylinder axis. For example, a three-dimensional selective RF pulse can excite a single voxel that is limited in all three spatial directions.
One important application of these multidimensional RF pulses is known as “inner volume imaging”. A multidimensional RF pulse is used as an excitation pulse. Its limited excitation volume allows the field of view (“FoV”) to be chosen smaller than the examination subject, without aliasing artifacts being created. A second important application is known as the “navigator technique”. Here, for example, a cylindrical rod is excited with a two-dimensional selective RF pulse (known as a pencil beam excitation) through the diaphragm edge, and information for the detection of the breathing movement is subsequently read one-dimensionally along the cylinder axis. The excitation of the cylindrical rod and the data acquisition for this purpose take place repeatedly in different time segments within an imaging sequence in order to identically associate the data acquisition with gates, matching the movement, or in order to associate the raw data, or the image data reconstructed therefrom with a movement phase or position, and/or to correct the data.
A two-dimensional selective RF pulse or pulse arrangement is achieved by activating temporally varying selection gradients (i.e. matching gradient pulses) along the two selective directions of the RF pulse during the RF radiation. These selection gradients describe a trajectory in transmission k-space, which is designated in the following as a “transmission k-space trajectory”, or just as a “trajectory”. This transmission k-space trajectory determines in which k-space regions the RF energy is deposited (distributed) for excitation. Because the phase and envelope (amplitude) of the B1 field of the RF pulse are selected as a function of time, matching the selected trajectory through transmission k-space, a precisely defined spatial selection volume (also designated as an “excitation profile”) can be realized in the image domain (i.e. in geometric space).
In practice, only EPI trajectories (corresponding to the readout gradients in the echoplanar technique, abbreviated as EPI for “echoplanar imaging”) and spiral trajectories (likewise known from the readout gradients) are used. EPI trajectories are thereby preferably used for the inner volume imaging, and spiral trajectories are primarily used for pencil beam excitations.
A practical problem in the realization of multidimensional RF pulses is known as gradient delay times. These delay times lead to a time deviation between the intended gradient shape and the actual applied gradient field. The simultaneously radiated, temporally varying RF pulse shape is therefore not matched to the gradient field, and this leads to a distortion and deviation from the desired excitation volume. The cause of these delay times is system imperfections of the gradient coil system and additional gradient fields induced by eddy currents. The delay times for the at least two involved gradient coils are normally different. A more detailed discussion is found in the journal article “On spatially selective RF excitation and its analogy with spiral MR image acquisition” by Peter Börnert and Bernd Aldefeld, MAGMA 7 (1998), P. 166-178.
Given a one-dimensional selective RF pulse, the selection gradient is normally constant. A deviation between the B1 pulse shape and the actual applied gradient field due to the delay times consequently only occurs here at the beginning or at the end of the radiation. Since the deposited RF energy is normally low anyway at these time intervals, the gradient delay times here behave “docile”—they thus have only a slight effect on the selection profile.